1. Field of the Invention
The invention relates to piezoelectric resonators and somewhat more specifically to improved resonators for use in electronic filter circuits.
2. Prior Art
Piezoelectric resonators comprised of a relatively thin wafer of a piezoelectric material having at least one resonance electrode positioned on a major surface thereof and having wafer areas adjacent such electrode which is free of electrodes, along with a dielectric non-conductive film over the electrode and the adjacent electrode-less wafer surface areas are known, for example from U.S. Pat. No. 3,401,276 (which is incorporated herein by reference and provides further details concerning such resonators).
With these types of known resonators, electrodes are applied by vapor deposition so as to bring about a resonance frequency of the thickness shear modulation in the region of the resonator which is covered by electrodes, which is only 0.8 times the value of the resonance frequency of the resonator in the non-electroded resonator region. In this manner, the active region of the resonator, i.e., the region thereof which vibrates, is concentrated in the electrode-covered region of the resonator. In order to be able to maintain this favorable relationship between the resonance frequency in the electrode region of the resonator and the electrode-less region of the resonator and to attain a fine frequency tuning, a coating is additionally applied onto the resonator electrodes and is permitted to extend onto at least one side of the resonator wafer across the total surface of the resonator on this side. In this manner, the resonance frequency in the electrode region and the resonance frequency in the electrode-less region is lowered and the relationship between these resonance frequencies is, essentially, left unaltered. The earlier noted U.S. Pat. No. 3,401,276 suggests that the coating, which is provided so as to extend over at least one major surface of the resonator, be composed of a material which has a high surface resistance, for example, silicon monoxide or anodized (i.e., oxidized) aluminum or oxidized tantalum. In other words, the prior art suggest utilizing dielectric materials having a high Q-value. Otherwise, the resonance electrodes would again be enlarged in an undesirable manner.
However, the application of a dielectric material coating is technically relatively expensive and disadvantageous in that the properties of such coating materials are timedependent. In other words, undesirably large changes of important operational parameters of the resonator occur as a result of aging effects, recrystallization processes and the like.